1. Field of the Invention
This invention relates to an image forming apparatus, such as an electrophotographic apparatus, an electrostatic recording apparatus or the like, and more particularly, to an image forming apparatus having carrying means for carrying an image supporting member, such as paper or the like, and for conveying the image supporting member to an image forming position.
2. Description of the Related Art
In a color electrophotographic copier, a transfer drum is configured by winding a carrying member, generally in the form of a carrying sheet such as a high-resistance film, around a drum-like frame. A transfer material of receiving an image is supplied to the transfer drum and wound therearound, and an end of the transfer material is mechanically fixed and held by a gripper disposed at a part of the circumferential surface of the transfer drum. In this way, an image supporting member (the transfer material) is carried by carrying means (the transfer drum).
In the process of rotating the transfer drum a plurality of times, a toner image having plural colors is electrostatically transferred from an image carrying member (such as a photosensitive drum) to the surface of the transfer material. For this purpose, a corona charging device is disposed behind the carrying sheet at the transfer position, or a conductive roller is disposed in rotatable contact with the transfer material held on the carrying sheet, and electric charges having a polarity opposite to that of the toner image are supplied to the transfer material. Subsequently, in order to discharge the transfer material from the carrying sheet, the electrostatic adsorption force (Coulomb force) produced between the transfer material and the carrying sheet is weakened using a corona discharger, and a separation claw is inserted between the transfer material and the carrying sheet to separate the transfer material.
In the transfer drum provided with the mechanical gripper, however, there are several disadvantages, For example, when the carrying sheet is formed in the shaped of a drum, the gripper portion distorts the drum causing imperfect cleaning of the carrying sheet. Further, the image, forming region is narrowed by the width of the gripper portion to produce an image-free area where an image cannot be formed.
Furthermore, when the transfer drum provided with the gripper is used, the position of the gripper on the transfer drum is fixed, even if both A3- and A4-format paper can be used as the transfer material carried by the transfer drum and the transfer drum has a circumferential length corresponding approximately to A3 format. Hence, when A4-format paper is carried on the transfer drum, a portion in which the transfer material is not carried on the transfer drum exists as much as about half the circumferential length. As a result, the image forming speed is reduced by that amount, causing an inefficient operation.
There has therefore been proposed a method of holding a transfer material (refer to Japanese Patent Public Disclosure (Kokai) Publication No. 55-32079 (1980)), in which a corona charger is provided facing the surface opposite to the transfer-material holding surface of the carrying sheet, and a conductive roller is also provided facing the transfer-material holding surface, to electrostatically adsorb (or adhere) and hold the transfer material relative to the carrying sheet.
In the above-described method of holding a transfer material, since the transfer material is electrostatically held relative to the carrying sheet, a mechanical configuration, such as a gripper and the like, becomes unnecessary. Thus, there is no hindrance for cleaning the surface of the carrying sheet, and problems, such as image-free areas and the like, can be avoided. The image forming speed can also be increased.
In the method of electrostatically adsorbing the transfer material on the transfer drum without using a gripper, when a toner image is transferred from the image carrying member onto the transfer material, there is an influence of the charged amount which has been given in order to absorb and hold the transfer material on the carrying sheet. The charged amount of the transfer material changes due to differences in relative dielectric constant according to the kind of the material. When the charged amount is small, adsorption and holding force on the carrying sheet is insufficient to cause a deviation in position, particularly, a deviation in position during transfer process. When the charged amount is large, transfer efficiency is reduced, causing the generation of an inferior transfer.
Furthermore, in the method of adsorbing and holding the transfer material on the transfer drum without using a gripper, when different kinds of transfer materials having different stiffness are used, the force with which the transfer drum adsorbs and holds the transfer material becomes weaker as the stiffness is larger, causing a deviation in position. The transfer drum can more easily adsorb and hold the transfer material as the stiffness of the transfer material is smaller. If the adsorption and holding force is increased too much, however, it becomes occasionally difficult to separate the transfer material from the transfer drum after image formation on the transfer material has been completed.